1. Technical Field
The present invention relates to an apparatus and method for measuring thickness and, in particular, to an apparatus and method for measuring the thickness of non-electrically conductive coatings on electrically conductive substrates and of electrically conductive materials, including electrically conductive coatings on nonelectrically conductive substrates.
2. Related Art
A number of types of coating thickness measuring instrument are known. One method used by such instruments to determine coating thickness is the so-called eddy current method. This method relies on the fact that the impedance of a coil carrying an alternating electric current varies in dependence on the distance of the coil from an electrically conductive surface. Instruments using this method typically include a probe including a coil. The probe is placed in contact with a coated conductive substrate, thereby placing the coil a predetermined distance from the substrate. An alternating current is applied to the coil and its impedance measured. This enables the distance from the coil to the substrate to be measured, from which the predetermined distance can be subtracted to obtain the thickness of the coating.
One way of measuring the impedance of a coil is to incorporate it into an LC oscillator the frequency of oscillation of which depends on the impedance of the coil. Impedance can then be determined from the frequency of oscillation. One apparatus using this approach is disclosed in U.S. Pat. No. 5,241,280.
A problem with the eddy current method is that the impedance of a coil is usually temperature dependent. Therefore, variations in ambient temperature can affect the accuracy of measurements made using a coil based measuring instrument.
U.S. Pat. No. 5,828,212 discloses a coating thickness measuring instrument including a magnetic field responsive sensor means, a measuring means for measuring the temperature dependent internal resistance of the sensor means and temperature compensation means connected to the measuring means for compensating for temperature drift of the measuring means. It appears from the disclosure that the instrument determines the temperature of the sensor and then makes an appropriate compensation based on the determined temperature. This is a rather complex approach because it is necessary to know the temperature response of the sensor in order to make an appropriate correction. Also, the temperature response of a sensor may change over time which cannot be compensated for by this instrument.
It is an object of the present invention to overcome, or at least mitigate, the above problems. It is also an object to provide an improved apparatus and methods for measuring thickness using the eddy current method which compensate for the effects of temperature.
It is another object of the present invention to provide an apparatus for measuring coating thickness which is sufficiently economical to produce so that it can be made available at a cost which is acceptable to an occasional lay user and which is able to indicate measured values in a way which is easy to understand by a lay user. Attempts have been made to provide a simplified instrument for use by lay persons, such as that described in U.S. Pat. No. 5,828,192, which discloses the use of a simplified display including three display elements to indicate the range into which a coating thickness measured by a conventional apparatus falls. Another such instrument is described in GB 2108672. Since both instruments employ conventional coating thickness apparatus they are restricted to measuring coating thickness on only ferrous or only non-ferrous substrates. Although instruments for measuring thickness on both ferrous and non-ferrous substrates are known, such as that described in U.S. Pat. No. Re. 35,703, they tend to be complex and costly and are therefore unsuitable for occasional use by lay persons. It is a further object to provide apparatus that is able to measure coating thickness on both ferrous and non-ferrous substrates.
It is a particular object to provide a low cost, easy to use, compact instrument for use in the automotive industry to detect if repair work has been carried out on automotive bodies which provides for some temperature compensation.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.
According to an aspect of the present invention there is provided an apparatus for measuring thickness including first and second inductors each forming part of a respective tuned oscillator circuit the resonant frequency of which varies with the impedance of the inductor, means for causing an alternating current to flow in each circuit, and hence inductor, means for measuring the frequency of oscillation of alternating current in each circuit and processing means, the inductors being arranged so that the first inductor may be positioned sufficiently close to a conductive surface so that its impedance changes and, when so positioned, any change in the impedance of the second inductor brought about by the surface is negligible compared to that in the impedance of the first inductor, and the processing means being arranged to determine a temperature compensated thickness measurement from the measured frequencies of oscillation.
According to another aspect of the present invention there is provided a method of measuring thickness including the steps of providing a first inductor forming part of a first tuned oscillator circuit the resonant frequency of which varies with the impedance of the inductor, positioning it close to a surface from which a measurement is to be taken, causing an alternating current to flow in the first circuit, measuring the frequency of oscillation of the inductor, providing a second inductor forming part of a second tuned oscillator circuit the resonant frequency of which varies with the impedance of the inductor, positioning it away, or otherwise isolating it, from the surface so that any effect the surface has on its impedance is negligible compared to any effect the surface has on the impedance of the first inductor, while exposing both inductors to substantially the same environmental conditions, measuring the frequency of oscillation of the second circuit and determining from the measured frequencies a temperature compensated thickness measurement.